The invention concerns an electrolytic source of high purity pressurized hydrogen.
Until the present time for the preparation of hydrogen in an electrolytic way, supplied under pressure higher than atmospheric, the filter-press construction of the cells connected in series is still used. Each individual electrolytic cell is formed by a pressure vessel divided into two parts by a vertical partition which ends above the bottom of the vessel to allow free motion of the electrolyte. In one part of the pressure vessel the negative hydrogen electrode dips into the electrolyte, in the other part the positive oxygen electrode. In the lid of the pressure vessel above the two compartments there are gas outlets for hydrogen and oxygen respectively provided by release valves. When voltage is applied to the electrodes the generation of the two gases occurs. The building-up of the gas pressure is achieved by closing the release valves, whereby it is necessary to draw the two gases proportionally in order to maintain equal pressure in both parts of the vessel. If this were not obeyed, the level of the electrolyte in the compartment with higher gas pressure would sink under the low edge of the partition and the two gases would mix up.
The disadvantage of these electrolyzers is the relatively complicated regulation and security systems for keeping equal gas pressure in the two parts of the cell; the greater the gas pressure the higher must be the accuracy of equalizing the pressures of the two gases. Another draw-back is the high content of oxygen in the thus produced hydrogen and vice versa. Moreover, both gases contain a high percentage of an aerosol of the electrolyte which must be additionally removed in separators. Another known device for the generation of pressurized hydrogen is an electrolyzer utilizing the diffusion porous pressurized hydrogen electrode the construction of which depends on the required hydrogen output.
While the electrolyzers for lower hydrogen output, up to several liters of hydrogen per hour, are formed by a single pressure vessel with small electrodes electrically connected in parallel, the higher hydrogen sources consist of a row of electrolytic cells connected in series. The electrolytic cells are formed by pressure vessels filled by an electrolyte into which dip the positive oxygen electrode and the negative reversible pressurized hydrogen electrode.
The electrolytic cells are further provided by inlets and outlets for a connection to the electrolyte circulatory system. The circulatory system, consisting of the electrolyte pump and the connecting, input and collecting channels, provides the supply of water to the electrolyte and its homogenization. However, at the same time it represents an inner electrolyte short-circuit of the electrolyzer cells. In consequence of this short-circuit there occur parasitic currents which have a negative effect upon the power output of the electrolyzer as well as, under certain circumstances, upon the purity of the generated hydrogen. The latter happens especially when the electrolyzer current is either switched off or reduced to a value near to that of the parasitic current. In that case in the hydrogen pipeline there appears an increased oxygen content which can mount up to several percent. The reason for this is that in the above quoted cases, due to the residual cell voltage, the parasitic currents, which are of a sign opposite to that of the generation current, produce an opposite polarization of the electrodes. This reversed polarization, especially marked in the case of a great number of cells, brings about a partial inversion of the function of the electrolyzer so that on the hydrogen electrodes oxygen is evolved while, on the other hand, on the oxygen electrodes hydrogen is produced, and this continues until the residual voltage of the electrolyzer falls to zero. For that reason in the electrolyzers of that type the generation current cannot be, as a rule, reduced under the limit of 30% in order to prevent the contamination of the generated hydrogen by oxygen. In case the electrolyzer is switched off and then on again, it is necessary to rinse thoroughly the hydrogen pipeline by hydrogen produced at the nominal output before pure hydrogen can be drawn.
The hydrogen electrodes of these types of electrolyzers consist of several layers. For the inner layers nickel and a special nickel catalyst are used in order to secure minimum overvoltage in hydrogen evolution. The external coating layer is usually made of copper because of the advantageous value of hydrogen overvoltage and because of applicability of the convenient technology of powder metallurgy. In an electrode manufactured in this way a preferential evolution of hydrogen on its inner layer is achieved. That is due to the average diameter of the pores of the inner layer between 0.04 and 0.06 mm as compared with the pores in the outer copper layer, smaller by an order of magnitude, about 0.003 mm. After the electrode is dipped into the electrolyte the pores of the external layer are closed by the capillary pressure which exceeds 100 kPa. In consequence, the hydrogen evolved in the inner layer does not penetrate into the electrolyte and can be drawn from the space of the inner layer under pressure not much higher than the 100 kPa. However, in reality some percentage of hydrogen evolves also on the outer copper layer and so, besides of the losses in hydrogen generation, there occurs the danger of formation of the explosive mixture in combination with oxygen. Besides, the efficiency of the described electrode decreases fairly quickly, be it due to the fatigue if the catalyst or to the deposition of impurities on the electrodes.
The above hydrogen electrodes represent a complex construction unit composed of several different materials differing in porosity, pore sizes, dimensions, shape, compressibility and even in thermal contraction in sintering. Especially the difference in contraction of the materials of the two adjacent layers of the electrode causes undesired deformations in the boundary regions during sintering. These deformations enlarge the pores of the outer layer which lowers the limit of the maximum attainable pressure of hydrogen.